1. Field of Disclosed Subject Matter
This disclosure relates to systems and methods for implementing a simple scheme of intelligent streak detection in constant velocity transport (CVT) image array scanning devices.
2. Related Art
A variety of systems and methods are conventionally used in digital scanning. Typical scanning systems employ full width imaging arrays that are stationary with respect to the devices in which they are housed, or which are caused to move relative to a stationary substrate to be scanned. In such systems, constant or variable rate movement is effected between the image to be scanned and the imaging array. The imaging array takes a rapidly sequential series of pictures of the image medium that is presented to be scanned as the relative movement is effected. The scanning system then digitizes the scanned image for storage and/or reproduction.
Typical full width imaging arrays include one or more rows of individually-pixilated sensors, each discrete pixel sensor being disposed to take the rapidly sequential series of digital images of the column of the scanned image which is presented within that pixel sensor's field of view.
A difficulty in such systems where the imaging point is stationary and the paper is moved past the sensor is that any imperfection in the field of view of the imaging array over a single pixel sensor or over a group of pixel sensors is repeatedly scanned and digitized as a dark or light spot in the scanned image. Relatedly, a defective sensor at a given pixel sensor location, in its failure to be able to scan image data to be digitized, will likewise potentially be digitized as a dark or light spot. The result is that the reproduced output image from the device can include a streak or line corresponding with the position of the imperfection or the defective sensor as the image of the dark spot is repeatedly reproduced to form the streak or line. Unlike a single pixel in the image being affected when the sensor array moves across a stationary image giving just a single or small group defect, a streak or line across the entire image is much more visible and objectionable.
Various exemplary scanning devices employ stationary what those of skill in the art recognize as Full-Width Imager Arrays (FWA) or linear Charge-Coupled Device (CCD) arrays. In such systems in which the imaging arrays are stationary, the arrays are often separated from the image medium or substrate to be scanned by a platen glass, or other transparent surface or lens, overlying the imaging array. Various exemplary methods are employed in these devices to move the image medium or substrate to be scanned across the transparent surface. One such exemplary method, commonly referred to as constant velocity transport (CVT) scanning, employs a belt, drum or other like device to move the image medium or substrate to be scanned across a linear sensor, such as, for example, an imaging array of individually pixilated sensors, and FWA or a CCD array.
As briefly discussed above, these imaging array scanning systems, including, for example, CVT scanning devices, may experience difficulties when dirt particles, stray paper fibers, contaminants, imperfections, obstructions and/or other objects, generally referred to throughout this disclosure as “obstructions,” appear on the transparent surface, or otherwise in the field of view of the imaging arrays. Such obstructions remaining stationary on the transparent surface, or otherwise in the field of view over the imaging array sensors, are repeatedly imaged and digitized as though it were part of the scanned image. The result is that the image reproduced by the exemplary CVT scanning device will include a streak or a line on the output image.
A variety of systems and methods are employed to ensure that a platen glass or other overlying transparent surface is kept free of obstructions. These systems and methods are directed to improving reproduced image quality by keeping the transparent surface obstruction free. Often the methods are manual, requiring visual inspection, and manual cleaning, of an exemplary system's platen glass or transparent surface. These methods can be mechanically automated in that a wiper blade, or the like, may be employed to routinely, automatically “wipe down” the platen glass or transparent surface. While limitedly effective in removing surface contaminants from a transparent surface, none of these methods account for obstructions or imperfections actually on an individual sensor, or for the potential of a defective sensor, providing the same sort of false scanned image data as an obstruction in the field of view of the array would produce on the overlying transparent surface. Also, obstructions can be on the underside of the glass or other hard to clean areas.
Other systems and methods for improving reproduced output image quality include automated system steps that review the reproduced output image in comparison with the scanned image in order to detect, and optionally compensate for, differences before the reproduced image is ultimately displayed, or transferred to an image receiving substrate for output. One complication with such an approach is that it is difficult to determine whether a line, which is digitized and reproduced as part of the output image, is actually part of the image that was scanned or whether it was produced because the imaging array repeatedly scanned an imperfection stationary in its field of view, or otherwise input false or non-image data from one or a series of defective sensors, and then reproduced the non-image data as a line or streak in the reproduced image.
Difficulties in detecting obstructions are particularly acute as elements of foreign matter which commonly produce streaks in conventional and exemplary CVT scanning systems are extremely small. Any “deposited” obstruction on the platen glass large enough to be detected by simple visual inspection is, most often, large enough to be picked up by the scanned image medium as the scanned image medium is moved across the platen glass or other overlying transparent surface. The obstruction is moved out of the way as being swept away with the movement of the image medium being scanned. Obstructions that are very small, on the other hand, have a greater tendency to adhere to the transparent surface and to resist being swept away by the movement of the scanned image medium.
Among other systems and methods employed to remove streaks from digital images produced in CVT scanning systems, software algorithms exist that review stored image data representing the scanned image in its entirety. Such software algorithms apply image data revisions before reproducing a digitally scanned image in an output display device. A disadvantage in such methods is that full scanned image review algorithms require the scanning of the entire image and storage of the data representing the entire scanned image before reviewing the data and applying necessary image data revisions. In high speed scanning applications, this requirement to record an entire image and then review and revise that image before reproducing the image has the potential to slow the scanning and reproduction process to an unacceptable level.
Despite the processing overhead, such systems are available. U.S. Pat. Nos. 6,393,161 and 6,522,431 describe variations in software systems for minimizing image defects in a hard-copy input scanner. These patents, which are commonly assigned to the Assignee of this application, commonly disclose scanners in which an image-bearing sheet is moved over a relatively narrow window, through which the image is recorded by a photosensitive chip. The patents specify the problem as involving conditions when “a spot of dirt attaches to the window, a streak results in the image data.” See Abstracts. In one instance, this streaking problem is addressed by detecting image data consistent with such a streak, and applying a suitable correction algorithm to the image data. The suitable correction algorithm is described as being chosen based on the thickness of the streak and the nature of the image data. In another instance, the streaking problem is addressed by having the scan line viewed by the photosensitive chip move a small distance within the window of the document handler, so that any spot on the window will not be repeatedly exposed by the photosensitive chip.
Separately, U.S. Pat. No. 7,528,997 discloses various methods for detecting streaks, such as streaks caused by imperfections in scanning systems that employ linear imaging arrays where the scanned image is sampled a minimum of two times by separate positionally discrete rows or sets of sensors in the imaging array. The at least two samples, though very close together, correspond to separate discrete linear scan locations. The scanned image data obtained from each of the two discrete rows is then normalized to adjust for different filtering capabilities, particularly color filtering in multi-spectral imaging arrays, in the discrete rows of the imaging array. The normalized scanned input image data from at least two discrete rows is then compared on a one-for-one basis in an attempt to detect imperfections overlying one or more rows of sensors in the imaging array.
U.S. Pat. No. 7,359,093 discloses systems and methods for streak detection in image array scanning using overdetermined scanners and column filtering. This streak detection method and system in a fixed imaging array digital scanning system obtains image data from each of the plurality of rows in the at least one full color spectrum channel set of rows of positionally discrete sensors and integrates this data to produce an estimate of image data recorded by at least one clear channel row of positionally discrete sensors. A clear channel error signal is generated by the comparison to alert the operator to the presence of non-image data. The clear channel error signal may be refined to through a low pass column filtering process in order to filter out potentially erroneous clear channel error resulting from thermal, mechanical or other noise sources unrelated to image scanning Stationary obstructions in the field of view of the imaging array, or defects in one or more sensors in the imaging array, are detected through this comparison which would otherwise appear repeatedly reproduced as streaks or lines in the reproduced output image.
Common to each of the above four patented methods of streak detection, isolation and/or correction are the complexity, computing overhead, additional expense, processing time and potential to miss the obstruction or defect.